The present invention relates to a communications system between a station on the surface of a liquid medium and submerged control and monitoring means of a group of submerged well head valves, such as those on oil or gas wells and which are remote from said station.
The invention more particularly applies to the control of the opening or closing of the valves of oil or gas well heads, which are submerged in a liquid, as well as to the monitoring of the operation of said valves from a control and monitoring station of the surface of the liquid medium.
A communications system between a station on the surface of a liquid medium and submerged means for the control and monitoring a group of submerged well head valves remote from this station is known. This system comprises a first communications channel having a first cable connected to the station and to the submerged control and monitoring means, as well as a second communications channel which can be a surface buoy or a cable close to the control and monitoring means. This buoy has means for hertzian communications with the station and it is connected by another communications cable to the submerged control and monitoring means. For each well head valve, said submerged control and monitoring means comprises at least one electrovalve for controlling the opening or closing of said valve, as well as sensors for monitoring the operation of said valves and the operation of the well head elements. The submerged control and monitoring means also comprise a submerged control and monitoring system connected to the electrovalves, to the sensors and to the two cables making it possible to establish communications with the station, either directly, or via the surface buoy. A communications system of this type is e.g. described in the journal Ocean Industry, February 1983, pp 47 to 50.
Generally, in systems of this type, the communications are synchronous, which is a serious disadvantage, particularly when there is a large attenuation of the amplitude of the control signals transmitted by the station to the submerged control and monitoring means. The loss of synchronization of the submerged control and monitoring means can lead to effects which are prejudicial to the satisfactory operation of the well head.
Another disadvantage of communications systems of this type is due to the fact that the choice of communications channel (cable or hertzian channel) is generally made on board the surface station. Switching from one communications channel to the other can take place at any time from the surface, because the switching relay on the ocean bed is controlled by a d.c. voltage from the surface and superimposed on the transmission by cable.
Another disadvantage results from the fact that the submerged monitoring and control means generally comprise a central unit and several peripheral units. This central unit permitting dialogue with surface equipments is an obligatory passage for the transmission to all the submerged equipments and is therefore highly prejudicial to the reliability of the system (common mode faults).